She was a New Jersey girl with Thai and Irish roots, a fashionista streak and a family full of university graduates. He was “rough around the edges,” he said, a Mexican-American, first in his family to go to college, a San Joaquin Valley transplant still trying to find himself.

“Everyone was like, 'Her? Him?’??” Solis said, now six years later. “But whenever we just let ourselves be, we said, ‘I don’t know what they’re talking about. We have more in common than they do.'"

More Americans are forming serious relationships across lines of race and ethnicity, moving in with or marrying people who check a different box on their census form. Married or unmarried, interracial couples were more than twice as common in 2012 than in 2000, U.S. Census Bureau data show.

Yet, not all kinds of relationships are as likely to cross those lines. Racially and ethnically mixed couples are much more common among Americans who are living together, unmarried, than those who have tied the knot, a Census Bureau analysis released last week shows.

Last year, 9 percent of unmarried couples living together came from different races, compared with roughly 4 percent of married couples. The same gap exists for Latinos — who are not counted as a race by the Census Bureau — living with or marrying people who aren’t Latino.

Earlier studies have shown that even among younger couples, Americans are more likely to cross racial lines when they move in together than when they marry. Scholars are still puzzling over why, musing that interracial couples might face added barriers to marrying — or might be less impatient to do so.

Some researchers believe the numbers are tied to continued challenges for interracial and interethnic couples in gaining acceptance from friends and family. Marriage can stir up a family’s disapproval in ways rooming together does not. Living together, “you don’t need to get a blessing from either side of the family,” said Zhenchao Qian, a sociology professor at Ohio State University. “Moving to the next stage is sometimes more difficult.”